# 555 alternate dual led string fader (breathing effect)

#### digital_citizen

Joined Jun 22, 2015
15
Hi I'm having some trouble in building a circuit based on one or max two 555 timers and at this point I don't know if it's even possible.
Basically I want to achieve a breating effect (slow on, slow off) of two strings of led (150mA max, so perfectly fine for 555 output) using a 555 timer and so far I've came across this schematic:

First of all this circuit doesn't have enough power, and second the fading effect is only during on time but not during off time (it does turn off abruptly). The supply voltage is 5V so I've added two transistors at pin3 (after the capacitor) but it doesn't work: I have an ugly effect.

So my question is: It's possible to achieve an alternating breathing effect with 555 or not?

#### dl324

Joined Mar 30, 2015
16,899
The general consensus is that it's better to use PWM (Pulse Width Modulation) to vary the brightness of LEDs.

Here's a waveform example:

A comparator is given as its inputs the triangle wave and the level voltage. Whenever the triangle voltage is lower than "level", the output will be HIGH. The frequency is determined by the square wave.

So you build something like a 1kHz square/triangle wave generator using a couple opamps. To adjust LED brightness, you change the "level" voltage. Higher means the LED is on more, lower means the LED is on less.

EDIT: You can change the polarity of what I described by swapping the comparator inputs or driving the LEDs high side instead of low side.

Schematic for the circuit is:

Originally discussed here.

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#### digital_citizen

Joined Jun 22, 2015
15
so with 555 is not possible to do that?

#### dl324

Joined Mar 30, 2015
16,899
so with 555 is not possible to do that?
It's possible to do PWM with a 555 timer, but it's easier to get precise control with the circuit I mentioned.

Here's a reasonable looking schematic:

From: https://electrosome.com/pwm-using-555-timer-ic/

You still need circuitry to generate the carrier abd and modulation waveform for the control input.

EDIT: correct typo

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#### ElectricSpidey

Joined Dec 2, 2017
2,770
EDIT:

Very sorry, but I had to withdraw the circuit...once again I missed the fact that the supply voltage was 5volts and not 12, the circuit I posted won't work very well at 5 volts.

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#### digital_citizen

Joined Jun 22, 2015
15
Here is a linear circuit I designed for a project I did a little while back, It will dissipate more power than PWM, but at 150 mA it's not really that much.

One LED brightens while the other dims. Remove the PNP section for a single LED.

View attachment 221798

Adjust as needed. (timing resistors and caps...etc)

The 1k on the output pin is not needed, but the feedback to the control voltage is. (also adjust as needed)
OK that's the closest thing I've been looking for: it's linear, uses one 555...but if it's not much is possible to omit the opamps, since I'm in a relatively hurry and don't have the time to order parts?

#### ElectricSpidey

Joined Dec 2, 2017
2,770
See Edit above.

Very sorry.

#### AnalogKid

Joined Aug 1, 2013
11,030
The post #1 schematic shows something that I guess is a yellow LED. Why is it there?

Usually, an LED breathing circuit has one transistor on the output of the 555. Here is a grab off the innergoogle.

UPDATE: Removed 2nd schematic. To have a complimentary LED, add a PNP transistor with the 2nd LED going to Vcc through its own current limiting resistor. Both bases connect to R1.

ak

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#### digital_citizen

Joined Jun 22, 2015
15
I want two alternating strings of LEDS that are fading on and off one after another (while one turns slowly on, the other turns slowly off, and so on). all the schematics you've posted are for just one led

#### ElectricSpidey

Joined Dec 2, 2017
2,770
Perhaps you could modify the circuits in post #8 by driving a PNP with the NPN, and put the second string on that.

I don't know, I don't have time to test.

Also, if you tie back the output to the control voltage you can change the crossover by modifying the threshold levels.

#### djsfantasi

Joined Apr 11, 2010
9,160
I want two alternating strings of LEDS that are fading on and off one after another (while one turns slowly on, the other turns slowly off, and so on). all the schematics you've posted are for just one led
I understood that’s what you wanted. But baby steps. You need to light ONE LED first, then TWO with the desired pattern. This circuit, with drivers, should light your LED strips as well as single LEDs.

You have been in competent hands, so I haven’t chimed in. But ElectricSpidey has a good idea. Basically, you need to invert one signal to get the second signal. Likely the LEDs have one side common, so you’ll need to switch either the high or low side. So instead of an NPN switching a PNP, consider one (NPN?) transistor switching one strip and a second transistor (NPN, as an inverter) that subsequently switched the second LED strip.

#### AnalogKid

Joined Aug 1, 2013
11,030
I want two alternating strings of LEDS that are fading on and off one after another (while one turns slowly on, the other turns slowly off, and so on).
See post #8 update.

#### ElectricSpidey

Joined Dec 2, 2017
2,770
I have tried this kind of configuration where the bases are connected together, it works but...there can be some undesirable results at the transition points.

#### digital_citizen

Joined Jun 22, 2015
15
I have tried this kind of configuration where the bases are connected together, it works but...there can be some undesirable results at the transition points.
Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off). Although I have to try this particular schematic that AnalogKid have posted and I will let you know very soon.

I understood that’s what you wanted. But baby steps. You need to light ONE LED first, then TWO with the desired pattern. This circuit, with drivers, should light your LED strips as well as single LEDs.

You have been in competent hands, so I haven’t chimed in. But ElectricSpidey has a good idea. Basically, you need to invert one signal to get the second signal. Likely the LEDs have one side common, so you’ll need to switch either the high or low side. So instead of an NPN switching a PNP, consider one (NPN?) transistor switching one strip and a second transistor (NPN, as an inverter) that subsequently switched the second LED strip.
Do you mean what exactly said AnalogKid?

#### djsfantasi

Joined Apr 11, 2010
9,160
Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off). Although I have to try this particular schematic that AnalogKid have posted and I will let you know very soon.

Do you mean what exactly said AnalogKid?
No, not at all.

Regardless of pull up/pull down/ base resistors, connect as follows (I don’t have tools at hand to draw a proper schematic)...
1. 555 to base of first NPN transistor (Q1)
2. First transistor emitter to ground; collector to led strip AND to second NPN transistor base
3. Second transistor emitter to ground; collector to second led strip

#### ElectricSpidey

Joined Dec 2, 2017
2,770
" Yes, I've tryed also some similar arrangement and it doesn't work properly (one led string doesn't turn off) "

Were you using two different color strips? For example if one is red and one is blue a diode in series with the red can help.

There can also be problems because the output of a bjt 555 is not symmetrical.

Also, when the bases of NPNs and PNPs are directly connected they can drive each other, and this must be considered in the design.

#### digital_citizen

Joined Jun 22, 2015
15
I'm using two identical strings like this:

So, what's the best idea then? I'm very confused now

#### djsfantasi

Joined Apr 11, 2010
9,160
I'm using two identical strings like this:
View attachment 221863

So, what's the best idea then? I'm very confused now
What’s the voltage required to light one string?

What current is drawn by one string when the above voltage is supplied?

#### AnalogKid

Joined Aug 1, 2013
11,030
First pass at a *concept* schematic. Note that no 1% parts are required; they are what's in my design libraries.

The voltage at node R1-C1 varies between 0.33Vcc and 0.67Vcc. The difference between max and min brightness depends on the LED forward voltage Vf, the transistor base-emitter voltage Vbe, and Vcc. The lower Vcc is, the greater the difference between max and min brightness.

A dual-opamp square/triangle wave circuit can give a much greater (and adjustable) difference between max and min brightnesses.

ak

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#### ElectricSpidey

Joined Dec 2, 2017
2,770
AK you made the same mistake I did...the OPs voltage is 5.